1. Field of the Invention
The present invention relates to a high frequency power amplifier circuit used in a transmitting stage of a radio communication apparatus.
2. Description of the Related Art
A transmitting stage of a wireless or radio communication apparatus may be enumerated for a typical application of the high frequency power amplifier circuit. The frequencies of the radio waves that are radiated from the output stage of the power amplifier circuit must be within those permitted by the regulations.
An example of the arrangement of one of the conventional high frequency power amplifier circuits will be referred to in brief. A signal is generated by a signal source, and enters an input terminal of the power amplifier circuit. The signal is applied to the base of a high frequency amplifying transistor of the npn type, by way of a first capacitor and a first impedance matching circuit. The emitter of the transistor is connected to a ground point. A first inductor for applying voltage to the collector of transistor is inserted between the collector of the transistor and the power source terminal. A collector potential of the transistor is outputted from an output terminal of the amplifier circuit via a second impedance matching circuit and a second capacitor. A second inductor for grounding the base of the transistor in a DC mode is inserted between the base of the transistor and the ground point. A first series resonant circuit containing an inductor and a capacitor that are connected in series, is placed between the base of the transistor and the ground point. A second series resonant circuit containing an inductor and a capacitor that are connected in series, is placed between the collector of the transistor and the ground point. A third capacitor for removing high frequency components is placed between the ground point and the power source terminal coupled with a first end of the first inductor.
It has been known that of the abnormal oscillating frequencies of the high frequency power amplifier circuit, the highest frequency against the frequency "f" of the input signal is half of the frequency "f". The oscillation at such a frequency is called an "f/2 oscillation (parametric oscillation)". The f/2 oscillation occurs under the condition that the real part of the input impedance to the amplifying element consisting of the transistor, for example, is negative at the f/2 frequency. To set up this condition, a series resonant circuit is coupled to the input circuit or the output circuit of the amplifying element.
In the conventional high frequency power amplifier circuit as mentioned above, the first impedance matching circuit and the second inductor, and the second impedance matching circuit and the first inductor make up respectively series resonant circuits. Provision of the first and second series resonant circuits in the base side and the collector side of the transistor provides a path through which an f/2 oscillation signal flows to the ground point, thereby to prevent the signals due to the abnormal oscillation other than predetermined frequencies being derived from the output terminal.
In the high frequency power amplifier circuit thus arranged, the series resonant circuit is connected between the base and the collector of the transistor as the amplifying element. In other words, it is directly coupled with the main part of the amplifier circuit. With this connection, the total number of the inductors directly inserted between the main circuit and the ground point are four; the first and second inductors, and the two inductors contained in the first and second series resonant circuits. The circuit inductance is increased, and the series resonant circuits become sensitive to input signals of the frequencies lower than the 1/2 frequency. Therefore, those circuits tend to oscillate in response to such input signals, viz., to cause the parasitic oscillation. Accordingly, the high frequency amplifier circuit is instable in operation.
Thus, the high frequency power amplifier circuit is provided with the series resonant circuits, which are provided in the main part of the amplifier circuit, in order to prevent the f/2 oscillation. Because of using the series resonant circuits, the amplifier circuit is instable against the input signal of the frequency below the f/2 frequency.